This is already done in the resource bind plugin. If you login a user second time using the same resource, the old one would be automatically logged out. If this is not good enough solution for you, than, as Steffen said, you need to implement a new processor. I would suggest to listen for resource bind XMLNS as this way you are sure that the login process has been successfully completed.

Just thinking, maybe a better solution would be to modify the code which disconnects the old connection for the same resource (in XMPPSession) to have an option for max number of user connections. When it is exceeded than the oldest connections would be dropped. Then when you set this to 1 it would work the way you want.